Question

What are octet and duplet rules? How do elements attain octet?

Answer 1

The study of electronic configurations of noble gases shows that they are chemically inactive because they have eight electrons in their outermost orbit except in the case of helium which has two. It suggested that they have stable configurations. In other elements, the number of electrons in their outermost shell is less than eight and hence they are chemically active.

Complete answer:
Inert gases or noble gases like Helium, Argon, Xenon, Radon are the most stable elements because atoms of these elements possess Octet structure which is having eight electrons in the outermost orbit. For example;
$Ne = 1{s^2}2{s^2}2{p^6}$ (having eight electrons in the valence shell)
The atoms of noble gases don’t combine with other atoms rather exist independently. The atoms of other elements which are not having octet structure try to attain eight electrons in their valence shell by combining with other atoms. The atoms combine in such a way that the electrons in the valence shell achieve octet or duplet structure of the nearest noble gas.
The atoms of different elements combine with each other either by transfer of valence electrons or by sharing of valence electrons in order to have an octet structure in their valence shells. This is known as Octet rule.
Also, electrons with lower atomic numbers try to attain two electrons that are duplet (instead of octet). This is called the duplet rule.
Elements attain octet or stability by losing electrons, gaining electrons or by sharing electrons.
For example; Sodium Chloride ( $NaCl$ ) is an ionic compound. In which sodium bears one positive charge $\left( {N{a^ + }} \right)$ and chloride bears one negative charge $\left( {C{l^ - }} \right)$. We know the configurations;
$Na = 1{s^2}2{s^2}2{p^6}3{s^1}$
$Cl = 1{s^2}2{s^2}2{p^6}3{s^2}3{p^5}$
If sodium can lose its outermost electron it’ll have eight electrons in the valence shell (octet). And chlorine has seven electrons in its outermost shell, if it gains one electron it’ll have eight electrons in the outermost shell (octet). Thus, sodium loses its one electron to chlorine forming $N{a^ + }$ and $C{l^ - }$ .

Note:
We can say that Noble gases are stable and inert because of their octet structures, while the other elements undergo bonding to attain stable octet structures and hence they are chemically reactive.